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AD621 Datasheet(PDF) 15 Page - Analog Devices |
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AD621 Datasheet(HTML) 15 Page - Analog Devices |
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15 / 20 page  Data Sheet AD8428 Rev. 0 | Page 15 of 20 INPUT BIAS CURRENT RETURN PATH The input bias current of the AD8428 must have a return path to ground. When the source, such as a thermocouple, cannot provide a current return path, one should be created, as shown in Figure 41. THERMOCOUPLE +VS REF –VS AD8428 CAPACITIVELY COUPLED +VS REF C C –VS AD8428 TRANSFORMER +VS REF –VS AD8428 INCORRECT CAPACITIVELY COUPLED +VS REF C R R C –VS AD8428 1 fHIGH-PASS = 2πRC THERMOCOUPLE +VS REF –VS 10MΩ AD8428 TRANSFORMER +VS REF –VS AD8428 CORRECT Figure 41. Creating an Input Bias Current Return Path INPUT PROTECTION Do not allow the inputs of the AD8428 to exceed the ratings stated in the Absolute Maximum Ratings section. If these ratings cannot be adhered to, add protection circuitry in front of the AD8428 to limit the maximum current into the inputs (see the IMAX section). IMAX The maximum current into the AD8428 inputs, IMAX, depends on time and temperature. At room temperature, the device can withstand a current of 10 mA for at least one day. This time is cumulative over the life of the device. Input Voltages Beyond the Rails If voltages beyond the rails are expected, use an external resistor in series with each input to limit current during overload condi- tions. The limiting resistor at each input can be computed using the following equation: MAX SUPPLY IN PROTECT I V V R − ≥ Noise sensitive applications may require a lower protection resistance. Low leakage diode clamps, such as the BAV199, can be used at the inputs to shunt current away from the AD8428 inputs and, therefore, allow smaller protection resistor values. To ensure that current flows primarily through the external protection diodes, place a small value resistor, such as a 33 Ω resistor, between the diodes and the AD8428. SIMPLE METHOD LOW NOISE METHOD +VS AD8428 RPROTECT RPROTECT –VS I VIN+ + – VIN– + – +VS +VS AD8428 RPROTECT 33Ω 33Ω RPROTECT –VS –VS I VIN+ + – VIN– + – +VS –VS Figure 42. Protection for Voltages Beyond the Rails Large Differential Input Voltage at High Gain If large differential voltages at high gain are expected, use an external resistor in series with each input to limit current during overload conditions. The limiting resistor at each input can be computed using the following equation: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − × ≥ G MAX DIFF PROTECT R I V R V 1 2 1 Noise sensitive applications may require a lower protection resistance. Low leakage diode clamps, such as the BAV199, can be used across the AD8428 inputs to shunt current away from the inputs and, therefore, allow smaller protection resistor values. AD8428 RPROTECT RPROTECT I VDIFF + – Figure 43. Protection for Large Differential Voltages |
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